In the microelectronics industry, the local oxidation of silicon (LOCOS) method is widely used for isolating MOS devices fabricated on a semiconductor substrate. By isolating the devices, the operational characteristics of the various components fabricated on the semiconductor substrate are enhanced by reducing or suppressing various parasitic mechanisms.
In the well-known LOCOS method, a layer of anti oxidant material is provided over a relatively thin pad oxide layer formed on a silicon substrate and etched into a mask which prevents the growth of field oxide over the masked active region of the substrate. After the anti-oxidant material has been grown, a field oxide region is grown on the substrate surface, adjacent the anti-oxidant material mask, doped with impurities, and then annealed to cause the field oxide to grow on the silicon surface in the non-masked areas, thus providing the isolation regions.
The field oxidation step results in the lateral diffusion of a thin, wedge-shaped portion of the oxide layer between the edge of the anti-oxidant material mask and the underlying thin pad oxide layer. This lateral diffusion of the field oxide between the mask and the substrate is known as "birds beak" encroachment. Birds beak encroachment reduces the total active area available for the formation of MOS devices. It also causes semiconductor stress and other faults that may result in the propagation of defects in subsequently formed layers during fabrication. Further, the relatively high temperature product of the field oxidation process permits the earlier doped field oxide impurities to diffuse beneath the active area. This diffusion may reduce the operational voltage threshold of the subsequently fabricated devices and increase the parasitic capacitance of the subsequently fabricated devices.
After oxidation, the anti-oxidant mask is removed thus leaving a steep step at the mask/field oxide interface. The step height of the remaining field oxide layer is approximately 50% of the total field oxide thickness. A step of this height causes several problems. One problem is that the step acts as a reflection surface during subsequent fabrication steps. This reflection surface promotes masking problems such as gate narrowing, which in turn results in current leakage from subsequently formed devices. A second problem is that the resulting oxide step interferes with film deposition during subsequent process steps, causing subsequently deposited film layers to be applied unevenly. A third problem associated with the oxide step height is that conventional fabrication techniques require extra etching steps to remove the excess amounts of polysilicon or silicide films at the field oxide/mask interface. The extra etching steps cause pitting of the substrate material, thereby damaging the substrate.
To reduce field oxide encroachment, conventional methods require the growth of a relatively thick layer of anti-oxidant material to mask the active areas of the substrate. However, the thick mask distorts the crystal structure of the underlying substrate abutting the edge of the mask, which can result in the propagation of imperfections through the substrate where devices are to be later fabricated.
It is therefore evident that there is a need for an improvement in the generally accepted MOS semiconductor device fabrication process which will reduce field oxide step height without introducing any unnecessary complexity to the standard fabrication process.